Pulse fired burners are the type which operate between firing and non-firing phases on a cyclical basis, the firing taking place therefore in a series of "pulses" between non-firing. These types of burners are used to heat enclosures such as furnaces and may be incorporated in the wall of the furnace.
Pulse-fired burners usually comprise a combustion chamber for the combustion of fuel (main fuel) with forced-draught combustion air (main air), the chamber having an inlet to receive the main air which may be preheated when the burner, as is very common, forms part of a regenerator and an outlet for discharging the combusted fuel from the combustion chamber to the furnace to be heated. A fuel injector is provided for injecting main fuel (e.g., nautral gas) at a firing or main fuel rate into the combustion chamber during the firing phase, and a pilot burner is also provided for igniting the injected fuel.
The fuel injector takes the form of a pipe which is supplied with main fuel from a suitable source for injection, which takes place usually through a wall of the combustion chamber.
The pilot burner comprises a tube which is supplied with fuel at a pilot level rate premixed with air supplied at a pilot level rate, the pilot fuel/pilot air mixture (pilot flame mixture) being ignited by a ignition device, (for example, a spark ignition device), so that the pilot burner provides a pilot flame at the end of the tube for igniting the fuel injected by the injector. In this case the pilot flame is arranged to be situated at a point where ignition of the injected main fuel is possible.
Main fuel injected by the injector at the main rate is, after ignition, combusted with the main combustion air in the combustion chamber of the burner. As a result flame (main flame) is produced in, and ideally is confined to, the combustion chamber.
The duration of the firing phases (during which main fuel is injected by the injector) may be set by timers which turn the injector fuel supply on at the start of the firing phase and turn the injector fuel supply off at the end of the firing phase.
Alternatively, the duration of the firing phase may be set by a thermostat which turns on the injector fuel supply when the temperature in the furnace is below a preset level and turns the injector fuel supply off when the temperature has reached its preset level.
For safety reasons, a device is provided to check for the presence of the pilot flame at the end of the pilot burner or for the presence of the main flame in the combustion chamber. This device most usually takes the form of a sensor which reacts to certain frequencies of light rays emitted by the flames, (for example, ultra violet). Other types of flame sensing devices may be used, such as thermocouples. In any case, when flame is present the sensor provides a detection signal which serves to maintain the burner operational. Where an ultraviolet sensor is used as the detector, the wall of the chamber is provided with a viewing port situated so that any flame produced by the pilot burner or the main flame resulting from combustion in the combustion chamber is visible externally of the burner both to the naked eye and, more importantly, to the flame sensor which is positioned appropriately. If there is no flame present for detection during either the firing or the non-firing phase, the flame sensor will cease emission of its detection signal, and as a consequence the burner will be shut-down automatically. In this case the fuel supply to the injector will, if the burner is in a firing phase, automatically be turned off to prevent the production of any flame in the combustion chamber. In addition, in most conventional systems the fuel supplied to the pilot burner will also be turned off automatically to prevent the production of a pilot flame.
One problem with this type of safety control is that, as long as a pilot flame is present, the sensor will be satisfied, and the burner will be fired during the firing phase. This could lead to a potentially dangerous situation in the event that the main combustion air for combustion of the main fuel in the combustion chamber cannot for some reason reach the combustion chamber. This might occur as a result of a blockage, for example, in the heat storage bed of the regenerator (where the burner is incorporated in a regenerative heating system). In this case, if the main fuel is still being injected at the main rate, the combustion chamber of the burner, the furnace itself and any further burners may receive this "neat fuel"--which if ignited by accident could explode.
For this reason, pulse-fired burners are usually operated so that, during firing cycles, the pilot flame is extinguished (after the main fuel provided by the injector has been ignited). This can be done simply by turning of the pilot fuel supply. At the end of the firing phase, the pilot fuel supply is turned on again and is reignited.
Furthermore, while under normal circumstances the precautions described should ensure that, in the event that there is a failure of the main combustion air to reach the combustion chamber of the burner during a firing cycle, shut down will automatically ensue, the pilot tube will still be supplied with pilot air even during the firing cycle, as only the pilot fuel supply has been turned off. This, albeit small, supply of air may nevertheless be sufficient, where it impinges on the main fuel emitted by the fuel injector, to promote combustion of the fuel in the region of impingement so as to provide a small flame of sufficient stability to satisfy the flame sensor that a flame is present. Thus shut-down may not follow although the prevailing situation demands it.
In practice the pilot fuel supply will be turned on just before the commencement of the firing phase, reignition of the pilot fuel being necessary of course at this point, and the supply will be turned of just after the beginning of the firing phase.
Apart from imposing a requirement that the pilot burner be interrupted periodically, additional controls are necessary to ensure that the operation of the pilot burner is appropriately out of phase with the operation of the fuel injector.
These controls are subject to heavy use. Particularly in the case where the firing and non-firing phases are of short duration, these controls are overworked and may break down or not maintain the appropriate out of phase situation between operation of the pilot burner and the fuel injector. In either case disastrous results may ensue.